Presentation on theme: "Gideon Bella Tel Aviv University On behalf of the ATLAS collaboration ATL-PHYS-PUB-2008-002 ATL-PHYS-PUB-2009-078 Prospects of measuring ZZ and WZ polarization."— Presentation transcript:
Gideon Bella Tel Aviv University On behalf of the ATLAS collaboration ATL-PHYS-PUB-2008-002 ATL-PHYS-PUB-2009-078 Prospects of measuring ZZ and WZ polarization with ATLAS
Motivation Gauge bosons are mostly produced singly, in electron- positron or quark-antiquark annihilation with transversal helicities. Longitudinal helicity of W and Z is possible only in di-boson production or in top-decay. The longitudinal helicity states are related to the spontaneous symmetry breaking of SU(2) x U(1). Longitudinal W helicity has been observed so far in e ⁺ e ⁻→ W ⁺ W ⁻ and in top decays. Longitudinal Z helicity has not been observed. Polarization in WW and ZZ events can help in separating a Higgs signal from di-boson continuum. Deviation from the SM expectation is an evidence for new physics, e.g. anomalous triple gauge couplings. 4 September 2009 2 G. Bella DSPIN-09
Di-Boson production in lowest order 4 September 2009 G. Bella DSPIN-09 3 W,Z (‘) W, Z, γ W,Z No ZZ in SM! We consider only purely leptonic decays of W and Z
Angular variables (e.g. for W ⁻ Z) 4 September 2009 G. Bella DSPIN-09 4 ¯ θ*θ* q q W⁻W⁻ Z rest frame ℓ¯ ν W⁻ rest frame ℓ⁻ℓ⁻ ℓ Z rest frame + ¯
Decay angle dependence 4 September 2009 G. Bella DSPIN-09 5 For W: For Z: with: - transversal (left-handed) helicity - transversal (right-handed) helicity - longitudinal helicity spin-density matrix elements
Decay angle distributions in MC 4 September 2009 G. Bella DSPIN-09 6 Use high statistics generator level MC of pp at 14 TeV (Pythia, without detector effects) ZZ WZ
Atlas Detector at LHC 4 September 2009 G. Bella DSPIN-09 7 Weight: ~7000 tons ~10 ⁸ electronic channels ~3000 km of cables 2800 physicists
Lepton measurement Electrons:Muons: 4 September 2009 G. Bella DSPIN-09 8 Triggered at first level by EM calorimeter Measured by inner detector ( η,ϕ) and EM calorimeter (E) at |η|< 2.5 Energy resolution at 100 GeV: Triggered at first level by MS (Muon Spectrometer) Measured by MS and inner detector at |η|< 2.5 Energy resolution vs. p T :
Event Selection (100 fb ⁻ ¹ @ 14 TeV) 4 September 2009 G. Bella DSPIN-09 9 WZZZ 0.690.067 σ·BR [pb] (from MC@NLO) 3 ℓ’s with P T > 7 GeV, and | η| < 2.5 Missing E T > 25 GeV |M(ℓℓ)-91.2| < 12 GeV ≤1 jet, P T < 30 GeV 50 < M T (ℓν) < 90 GeV ∢ ( ℓν) T > 40 ⁰ 4 ℓ’s with P T > 7 GeV, and | η| < 2.5 ≥2 ℓ’s with P T > 20 GeV |M(ℓℓ)-91.2| < 12 GeV selection cuts: 28732194events selected: 0.40%3.3%efficiency tt, WW, ZZ, Z+jet, Ztt, Zbbbackgr. sources 1%<1%backgr. level
Event reconstruction 4 September 2009 G. Bella DSPIN-09 10 All 4 leptons measured ZZ WZ 3 charged leptons measured. P T ( ν) is the missing E T of the event. P L ( ν) can be calculated from W-mass constraint – a quadratic equation with 2 solutions. We used the weighted mean of both solutions, with the theoretical SM cross- section used as the weight.
Resolution in WZ reconstruction 4 September 2009 G. Bella DSPIN-09 11 WZ invariant mass - ŝ W ⁻ decay angle
Extraction of ρ ₀₀ from cosθ* ℓ distribution 4 September 2009 G. Bella DSPIN-09 12 ZZ Use projection operators: Λ Τ = Λ₋₋+Λ₊₊ = 5cos²θ* ℓ - 1 Λ L = Λ₀₀ = 2 - 5cos²θ* ℓ The sum is over all data events (N) in a given ŝ bin. is the detector correction factor, calculated from MC for bin j in ŝ and | cosθ* ℓ |, populated by event i (4x10 bins).
Extraction of ρ ₀₀ from cosθ* ℓ distribution 4 September 2009 G. Bella DSPIN-09 13 Decay angle distributions Z in W ⁻ Z WZ W⁻W⁻ W⁺W⁺ Use event-by-event ML fit to the theoretical distribution, corrected for detector effects, to obtain ρ₀₀ in 3 different ŝ bins for Z, W ⁻, W ⁺.
Systematic effects 4 September 2009 G. Bella DSPIN-09 14 Parton Distribution Functions. for ZZ – use different sets (CTEQ, EHLQ2, MRST) for WZ – use 40 CTEQ6M error sets. (for WZ only) Theoretical cross-section used for the weighting of the two solutions from the kinematic fit. Use cross-section with anomalous couplings within the Tevatron limits instead of the SM one (dominating). MC statistics Total systematic errors are typically (20 – 50)% of the statistical ones.
Results 4 September 2009 G. Bella DSPIN-09 15 ZZ WZ ATLAS Preliminary
Conclusions 4 September 2009 G. Bella DSPIN-09 16 Z and W polarization in ZZ and WZ events can be measured, but needs at least 100 fb ⁻ ¹ (not for next year…) Dependence on ŝ can be observed for ZZ. Fraction of longitudinal polarization is expected to drop with increasing energy (W and Z become more “photon like”). Error is dominated by statistics. Better accuracy is expected at the super-LHC